Production of polarizing elements



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INVENTOR. @0f/60 ll/l//no'or @row/1 ATTO RNE YS Dec. 13, 1966 A. w.BRowN 3,291,353

PRODUCTION OF POLARIZING ELEMENTS MX B\NDER NGRED ENTS DEAERATE MD(GLASS FLAKES WlTH DEAERATED BlNDER SHEET THE BNDER- FLAKE MD( HOT DRESSTHE SHEETS To EFFECT CURE 0F B\NDER HEAT THE CURED SHEETS T0 DISTURBRES\N-GLASS TNTERFACES INVENTOR. AHred Wmsor Brown QAM Afrox-:News

ee. 13, 1966 A. w. BROWN PRODUCTION OF POLARIZING ELEMENTS OriginalFiled July 30. 195B 4 Sheets-Sheet 3 I/I l 11d l I) 'III' Il 35 l l l l1 l l l l 1 1 BMU mi- Fmg..

INVENTOR. A\Fred Wmsor Brown ATToRueYs Dec. 13, 1966 A. w. BROWN3,291,868

PRODUCTION OF POLARIZING ELEMENTS Original Filed July 30. 1958 4Sheets-Sheet 4.

/Qf/lea' Winsor Brown Wil@ 4 f @wl HTTOPNE Y6 United States Patent O 3291 86S PRODUCTION F POLPJZING ELEMENTS Alfred Winsor Brown,Woonsocltet, RJ., assigner to Owens-Corning Fiberglas Corporation, acorporation of This application is a continuation of 751,933, filed onJuly 30, 1958, now abandoned, which is a continuationin-part of 627,980,filed on December 13, 1956, now U.S. Patent 2,981,480, which is in turna continuation-in-part of 596,397. filed on July 9, 1956, now abandoned.

This invention relates to the production of polarizing elements, and,more particularly, to the production of material which is polarizing byvirtue of the incorporation therein of glass in the form of flakes oriiakelets.

Various naturally occurring and synthesized materials are polarizing, ororienting, to light waves. A wave of ordinary or unpolarized light isconsidered as involving vibrations in random directions lying in a planeperpendicular to the direction of propagation of the wave. Polarizedlight may be considered as a fraction of ordinary light, being the partof the normal vibration which lies, also, in a plane through the axis ofthe wave. It has been emonstrated that polarized light, when produced byfiltering the random vibrations which do not lie in the second identiedplane, can constitute not more than 50 percent of the wave energy of theoriginal or unpolarized light.

The large scale production of polarizing elements was first madepossible by the discovery that minute crystals of a polarizing materialcan be incorporated in a plastic sheet material, suitably oriented, andthen formed into polarizers of desired shapes.

A more recent development (see U.S. Patent 2,402,176 and ScienceIllustrated, volume 4, No. 6, I une 1949, pages 124-27) is based uponthe discovery that polarizing elements can be produced from amultiplicity of layers of very thin plastic material suitably adheredinto the polarizing element. This recent development provides, from manystandpoints, a substantial improvement in the production of polarizingelements. There are however, difficulties from the processing standpointas will be inimediately apparent from a consideration of therequirements such material must meet. For example, the individual sheetsof a plastic material must have a maximum thickness of about 0.004",preferably a thickness of 0.0015 or less, and from 15 to 30 such sheetsmust be formed into the polarizing element. The sheets must be separatedfrom one another by a distance greater than the wave length of the lightto be polarized, but not more than about 0.010", and a substance(usually air) having a refractive index differing from that of thesheets by at least 0.2 must be disposed between adjacent sheets. Thenecessary spacing can be achieved by means of spot adhesive or ofsuitable spacing means at the edges holding the entire assembly intransverse tension, air or gas being included and sealed between theadjacent sheets.

Methods for producing glass in the form of akelets are known, and suchmaterial, refractive index 1.55, is presently available in substantialquantities. The production of papers therefrom has been suggested, butthe product possessed no tear strength (see Properties of Paper MadeFrom Glass Flakes, Callinan and Lucas, Report of N.R.L. Progress, May1955). Impregnation of the paper with various binders was suggested toprovide a tear strength that would make feasible the use of the productin contemplated applications.

It has been found that a polarizing element somewhat similar to thatdisclosed in U.S. Patent 2,402,176 can be ice produced by dusting glassfiakelets onto a transparent plastic sheet. Either a transparent or atranslucent (lightdilfusing) sheet can also be placed above theflakelets. If desired, the edges of the two sheets could be sealedtogether with any suitable adhesive in order to form a unitary sandwichstructure which would constitute a usable polarizing element. As hasbeen stated above, the glass akelets have an index of refraction ofabout 1.55. Air, which separates the flakelets in the structuredescribed, has a refractive index of 1.00. As a consequence, suchstructure comprises a mass of glass flakes supported in spacedarrangement in the mass, and a substance (air) having a refractive indexdiffering from that of the glass by at least 0.2 disposed betweenadjacent liakelets. A similar polarizing element is produced bypositioning a sheet of paper made from glass flakelets on a transparentsheet in a like manner. Such polarizing elements, while not specificallydisclosed in the prior art, are ditlicult to produce, and are notclaimed herein. Neither a prior art paper produced from glass fiakeletsand impregnated with a binder nor a sheet of transparent materialreinforced with glass akelets constitutes a polarizing element becausethe requisite difference in refractive index between glass and binderdoes not exist.

The present invention is based upon the discovery of polarizing elementswhich can be produced with facility from thin plates or iiakelets ofglass. The glass flakelets are bound in place in the polarizing elementsby a suitable binder.

One object of the instant invention is, therefore, the production ofpolarizing elements comprising glass flakes or akelets.

Another object of the invention is the provision of irnproved polarizingelements.

A further object of the invention is to provide an improved method formaking polarizing elements.

Other objects and advantages will be apparent from the description whichfollows, reference being made to the accompanying drawings, in which-FlG. l is a perspective view showing a polarizing sheet produced by themethod of the invention;

FIG. 2 is a vertical sectional view along the line 2 2 of FIG. 1 showingdetails of the structure of the polarizing sheet;

FIG. 3 is `a View in vertical section similar to FIG. 2, but showing adifferent polarizing sheet structure;

FIG. 4 is a iiow diagram representing the various steps in a method forproducing polarizing panels, and shows, specifically, the steps in thebest presently known mode for practicing the method of the invention;

FIG. 5 is a perspective View showing apparatus in which a deaerationstep in the method of FIG. 4 can be carriedout;

FIG. 6 is a view in perspective showing apparatus in which a mixing stepof the method of FIG. 4 can be carried out;

FIG. 7 is a schematic View in elevation representing apparatus in whicha sheeting step of the method of FIG. 4 can be carried out;

FIG. 8 is a partially schematic View in vertical section of a moldingpress in which a hot pressing step of the method of FIG. 4 can becarried out;

FIG. 9 is a partially schematic view in vertical elevation showingapparatus in which a final post curing heat treatment step of the methodof FIG. 4 can be carried out;

FIG. l0 is a view in front elevation showing a light polarizer anddiffuser fabricated from polarizing elements produced according to theinvention;

FIG. 11 is 'a sectional View along the line 11-11 of FIG. 10; and

FIG. 12 is a partially schematic view of a modified polarizer anddiffuser which includes a pa'nel produced according to the invention.

A new polarizing element is provided according to the invention. Suchpolarizing element comprises a mass of glass ilakelets supported inspaced arrangement in the mass by a suitable binder, and a substancehaving a refractive index differing from that of the glass by at least0.2 disposed between adjacent flakelets.

As Will subsequently be discussed in more detail, the substance having arefractive index differing from thatpof the glass by at least 0.2, whichsubstance is disposed between adjacent llakelets, can be .air oranotherl gas, can be a. coating applied to the 'glass flakelets, or canbe a portion o'f the binder material itself. l

The binde can be any suitable organic or inorganic, natural or'synthetic adhesive composition. It is preferred that thebinder be atleast virtually transparent in order to avoid absorption, by thepolarizing element, of any [appreciable portion of transmitted light. Iolyvinyl acetate, polyvinyl pyrrolidone, polyvinyl methyl rnethacrylate,methyl methacrylate and other acrylic binders, polystyrene, and variouspolyesters can be mentioned as specific suitable organic binders thatare at least virtually transparent.A Silicic acid, magnesium silicate,and silicates of other metalsforming oxides, hydroxides and carbonateshaving a pH not greater than 10.5 can be mentioned as specific inorganicbinders. Neither the strength characteristics, within reasonable limits,nor the chemical identity of the binder employed in a polarizing elementaccording to the invention is of particular importance. The solefunction of the binder is to support the flakes or platelets of glassrelative to one another. If greater strength than is provided by thebinder is required, the polarizing element can be positioned on asupporting sheet, for example of glass or of a transparent ortranslucent sheet material, or between two such sheets. Such sheet orsheets can provide the required strength, while the element according tothe invention is polarizing to light transmitted therethrough. Ifdesired, at least one such sheet can be diffusing t-o light.

One method for producing a light polarizing element according to theinvention comprises forming a uniform suspension of a limited amount ofa suitable binder, a solvent or dispersing agent therefor, and glassflakes or platelets, for example in a paper beater, casting theresulting suspension on a suitable foraminous base, for example ascreen, allowing excess binder and solvent or dispersing medium to drainthrough the foraminous support, and hardening the binder. Only a limitedamount of the binder should be employed to produce la polarizing elementaccording to this method when presently available glass ilakelets areemployed. As has been indicated above, such akelets have a refractiveindex of about 1.55, which is substantially identical with therefractive indices of most binders. Therefore, if a large amount of thebinder is employed, so that the space between adjacent llakelets iscompletely filled with binder, the resulting paper has a substantiallyconstant refractive index throughout its thickness, and does not act asa polarizing element.

In a specific instance, a polarizing element has been produced in themanner described in the preceding paragraph, using water as a dispersingmedium, from glass ilakelets and 3 percent of polyvinyl acetate as abinder, based upon the total weight of glass and binder. In general,'from about 1 percent to about 5 percent of a binder, based upon thetotal weight of glass and binder, can be employed to produce such apolarizing element. In view of the foregoing discussion it will beappreciated that the ability of such, papers to polarizelight is,attributable to the presence of a gas, usually air,r between adjacentflakelets, or between flakelets and binder. The refractive index of thegas is Iabout 1.00, and thatof the binder and glass about 1.55. Usingsuch glass flakelets, the production of a paper with more than aboutpercent of a binder, based on the weight of binder and glass, results ina` paper having relatively few gas-filled voids, and, as a consequence',relatively few discontinuities in refractive index. Such a paper iscomparative ineffective as a polarizing element, and is completelynon-polarizing if all such gas-lled voids are eliminated. A polarizingelement which has been produced using less than about l percent of abinder, on the indicated basis, is comparatively unsatisfactory becauseof its relative weakness.

When it is desired to produce a polarizing element accor'ding to theinvention in the form of a paper, and using more than about 5 percent,on the indicated basis, of a binder, an additional step, beyond thosediscussed above, is required. For example, `a coating of high refractiveindex can be applied to the glass flakelets; a controlled amount of airor other gas can be incorporated into the slurry; a vaporizable materialsuch as a volatile solvent can be incorporated into the slurry andvolatilized after formation of the paper to force a separation betweenthe binder and the flakes; or the binder can be treated to effectgassing of a portion thereof in order to force a separation between thebinder and the flakes. A polarizing element according to the inventioncan also be produced, for example in the form of a paper, from glassflakelets having a refractive index differing from that of the binder byat least 0.2, preferably by at least 0.5.

In general, when glass flakelets produced from a glass having arefractive index differing from that of the binder are employed toproduce a polarizing element, it is preferred that the refractive indexof the glass be higher than that of the binder. In such instance, alight wave passing through the polarizing element is subjected to therequisite change in refractive index in passing from glass to binder orair, in passing from binder to glass or air, and in passing from air tobinder or glass. A polarizing element produced from glass having a highrefractive index, for example approximately 2, need not contain pocketsof air or other gas, as the binder itself can constitute a substancedisposed between adjacent flakelets, and having the requisite differentrefractive index; the same is true of polarizing elements produced fromflakelets coated with a material of high refractive index. Where apolarizing element is produced either using glass flakes having a highrefractive index or relatively low index flakes coated with a materialof high refractive index, it has been found that optimum results areachieved by using a mixture of the coated flakes or the high refractiveindex flakes and uncoated flakes having a refractive index of about 1.5.Preferably, the polarizing sheet which is produced should contain fromabout 10 to about 15 interfaces of high refractive index, whether coatedor continuous, and most desirably about 12, in a thickness. In this way,panels having an optimum combination of polarizing and transmissioncharacteristics are obtained. A panel having a practical ratio of glassflakes to binder, and only about the indicated number of interfaces in agiven thickness, is extremely thin and, therefore, relativelyunsatisfactory for most purposes. Such a thin panel can, however, belaminated with one or more additional panels, such addit1onal panelspreferably being reinforced with glass flakes having a refractive indexof about 1.5, and without appreciable disturbance at binder-flakeinterfaces, to produce a composite polarizing panel according to theinvention. If desired, a mixture of high index flakes and lower indexflakes of about 1.5 can be mixed in such proportions that, in a panel ofa desired dimension, on the average, the indicated number of high indexflakes will be found in a given panel thickness.

A coating of high refractive index can be applied to glass llakelets inany of several ways. For example, titania, zirconia, or like materials,can be deposited on the glass surface, as in a drum tumbling devicecontaining glass flakes and such a material. The surfaces of the glassflakes can .also be treated with titanium tetrachloride, and then withammonium hydroxide, and pyrolized, for example at about 600 F. TiOg isformed on the glass the exterior thereof.

surfaces as a result of such treatments. The glass surfaces can also beheated and exposed to a gaseous atmosphere of tetraisopropyl titanate,or tetraisopropyl titanate can be dissolved in a suitable solvent,applied to the glass surfaces, and a titanium silicate coating formed byheating.

One method for producing glass flakelets involves flowing a stream ofmolten glass through an annular orifice in the form of a tube, grippingand advancing the tube with attenuating rolls disposed below theorifice, and introducing air or other gas into the interior of the tubeunder slight pressure to prevent collapse of the walls thereof, or tocause slight expansion thereof. A coating of high refractive index canbe applied to the interior of the tube by using isopropyl titanate as atleast one component of a gas which prevents collapse of the walls of thetube in such process. The coating would then be only on the innersurfaces of the tube, and on only one side of the glass flakelets formedby cracking of the tube. However, if desired, the tube could also besuitably enclosed and an atmosphere of tetraisopropyl titanatemaintained on Glass flakelets formed by cracking such a tube would thenhave the coating on both surfaces thereof.

It will be appreciated from the foregoing discussion that, in manyinstances, a layer of air or other gas, disposed between at least oneside of a flake .and a binder supporting that flake relative to theremainder of the element, is advantageous in polarizing elementsaccording to the invention. The provision of such a layer of air orother gas can be facilitated by employing glass flakelets that are notplanar, but have a certain curvature. Glass flakelets having a curvaturecan be produced in generally the manner described 'above by eliminatingthe pressure of air or other gas inside the tube so that the wallsthereof are collapsed by the pull of the attenuating rolls.

Two part flakes, hereinafter `for convenience referred to as doubleglass flakelets can also be produced as described in U.S. Patent2,457,785. Such flakes result from collapse of the glass tube betweenattenuating rolls, and cracking of the resulting film. The double glassflakelets are composed of two -bodily separate ilakelets which aretightly held together, probably by Van der Waals forces.

It will be appreciated that the thickness of the glass flakelets can bea significant factor in determining the characteristics of polarizingelements according to the invention. Satisfactory results have beenachieved using commercially available flakelets having an average ornominal thickness of about 0.00004 to 0.00038, and probably varyingseveral fold from such average. The average thickness lof such flakeletscan be increased or decreased, however, by decreasing or increasing therate at which the attenuating rolls discussed above are driven. Thethickness of the flakes or platelets of glass can, therefore, be variedwithin substantial limits either above or below the presently`commercially available 0.00004 to 0.00038 thickness. Presentlypreferred flakes have a nominal thickness of 0.00008. -It has been notedthat extremely thin flakes of glass, e.g., 0.00004 to 0.00008, per seshow an interference pattern.

It has been found that air clings to the surfaces of glass flakes orplatelets to an extent such that, if the flakelets are merely mixed witha suitable binder and the binder cured, air bubbles will cause theresulting cured mass to be opaque, or, at best, only translucent, eventhough both the binder and the glass are transparent. Various techniqueshave been tried to eliminate such air from binderglass ilakeletmixtures, prior t-o curing of the binder, because the entrapped airinterferes with reinforcement of the binder by the flakes, as well aswith light transmission. For example, glass flakelets have been mixedwith monomeric styrene, in the absence of any polymerization lcatalystfor the styrene, and subjected to heat under vacuum in order toeliminate the entrapped air.

A polarizing element in the form of a hardened sheet material reinforcedwith glass flakelets |can also be produced by any of several othermethods. For example, lglass flakes can be mixed with a suitable binderresin, placed under vacuum to remove entrapped air, and the mixtureplaced between heated platens to form a cured sheet of desired form. Thecured sheet can then be converted into a polarizing element `by causinga disturbance at some of the interfaces between the glass and thebinder. Such a disturbance can be caused by subjecting the sheet tomechanical shock, to high frequency electrical waves or to other highfrequency vibration, or to thermal shock. In addition, such elements canbe produced by curing the sheet at a temperature above the maximumcuring temperature for the binder t-o cause rapid curing or over-curingthereof, and resulting disturbance at some of the interfaces. At least apart and preferably only a part of the flakes, before mixture with thebinder, can also be treated with a material which provides a nonadhesivesurface, for example a stearylsilane. Preferably, such a material isapplied to some of the flakes from a solvent solution, and the flakesare only partially dried, so that some of the solvent remains and isvaporized during curing or during a post-curing heat treatment. Curing asheet containing such flakelets produces a structure which is polarizingbecause of interfacial disturbances caused by lack of adhesion betweenthe binder and the coating, at least when cure is at a comparativelyhigh temperature for the particular binder, or when the sheet, aftercuring, is reheated as subsequently discussed. Also, at least a part ofthe flake can be moistened, mixed with a binder, the mixture formed intothe desired shape, and cured in a dielectric oven so that the moistureis vaporized, causing the desired disturbance at the interface betweenflake and binder. Moistening of the flake can be accomplished by use ofliquid water, or in a high humidity atmosphere. Any suitable solvent canbe used instead of water to m-oisten the flakes, or can be mixed withthe binder in order to produce a lsimilar gassing effect and resultingdisturbance during curing. A gassing agent, such as monomeric styrene,an acrylic monomer, or the like, can also be mixed with a suitablebinder. Usually, a suitable gassing agent is capable of reaction withthe binder, e.g., a polyester; in such case, an excess of the monomer isused, and the excess acts as the gassing agent. Flakes can then be mixedwith the binder composition, formed, and cured. The presence of thegassing agent also causes bubbling during curing, or during a heatingstep which follows curing, and disturbance at the binderflake interface.Splitting a panel composed of glass flakes embedded in a binder causes amechanical shock at the binder-glass interface, thus making the panelpolarizing, and also produces rough diffusing surfaces.

As a specific example, a polarizing element according to the inventioncan be produced from parts of a commercially available polyester resin,10 parts of monomeric styrene, l part of benzoyl peroxide, and 50 partsof a -ake glass. The various ingredients are mixed until the flakes arewet by the resin; the mixture is then subjected to vacuum to remove mostof the entrapped air, either during mixing or subsequent thereto; andsheets or other shapes are then formed by curing the mixture at about200 F. between heated platens. The platens may be of such configurationto produce a flat sheet, a corrugated sheet, or any other desired shape.A flake glass paper of the type previously described can, if desired, beattached to the surface of the cured sheet, or positioned between theheated platen and the curing resin so that it will be adhered to thefinal cured sheet. The described composition of binder, styrene, benzoylperoxide, and flake glass can also be extruded in any desired shape, forexample as rods, tubes, sheets or other, and subsequently cured whilesuitably contained under pressure. The resulting cured product can thenbe converted to a polarizing element by means of mechanical or thermalshock, or by means of high frequency electrical Waves or mechanicalvibration. Such a sheet is indicated generally at l in FIG. l, and iscomposed of a suitable binder l1 with glass flakelets 12 embeddedtherein. As can be seen in FIG. 2, the resin il constitutes a continuousphase with the flakelets embedded therein and bonded thereto, anddisposed with their major surfaces generally parallel to the majorsurfaces of the sheet 10. By virtue of the thermal shock to which thesheet was subjected, as described, pockets f3 of air or other gas aredisposed between the major surfaces of the flakelets l2 and the body ofth eresinous binder lll. In this structure, the pockets 13 constitutethe substance of refractive index different from that of the flakes.

An improved method of forming a polarizing element involves theutilization of multiple heating steps to cure the binder and at the sametime to disrupt the interfaces between the `flakes and the binder. Theinitial heating of the resin and the flake mix at least partially curesor sets.

the resin and the subsequent heating step or steps disturb the resin toglass interfaces to provide polarization.

A polarizing element according to the invention has also been producedfrom a mixture of one part of a commercially available coupling agent,80 parts of double glass akes and parts of a commercially availablepolyester resin. The mixture was cured, in the form of a sheet, underpressure in a suitable mold, for ten minutes at 150 F. and then fortwenty minutes at 300 F. The resulting sheet was found to be polarizing.It will be noted that a high temperature cure was employed, as well asdouble glass iiakelets. It is believed that both of these factorscontributed to polarization, the former for reasons previouslydiscussed. The double akes are separated from one another at least atsome points, by a thin film of air, so the necessary refractive indexdifference can be achieved therefrom. Another example of a polarizingelement according to the invention is produced by curing in thedescribed manner a mixture wherein single tiakes are substituted for thedouble glass iiakes.

The following compositions have been mixed and cured to form apolarizing panel:

Example 1 Parts by weight Methyl methacrylate monomer 70 Polyester 70Benzoyl peroxide catalyst 2.8 Untreated, single platelets of glass 70Example 2 Parts by weight Styrene monomer 70 Polyester (P-433, Rohm &Haas) 70 Benzoyl peroxide catalyst 2.8 Untreated, single platelets ofglass 70 Both of the two above formulations were thoroughly mixed in aHobart, high-speed mixer wherein the -akes were substantially brokenwith the resut 4that a final flake or platelet size ranging from JAG to1/8 appears to be produced. The platelet size should be from about 30 toabout 120 mesh when producing either reinforced sheet or polarizingpanels. Larger liakes make it diicult to obtain a uniform dispersion offlake and binder. After thorough mixing of the ingredients, thecomposition was subjected to a vacuum (minus 30" of mercury) for tiveminutes to remove entrapped air. The resulting deaerated composition wasthen formed into sheets by placing the composition between sheets ofcellophane and spreading the composition between these cellophane sheetswith a roller to form a sheet about 0.06 thick. An appropriate jig wasused to provide the proper sheet thickness. Overfeeding of properlyspaced rolls may also be used to advance a sheet `of the properthickness therebetween. The resulting sheets were heated in an oven atabout 250 F. to cure the resin.

Another method of forming a sheet comprises introduc- 8 ing thecomposition between the platens of a press. A total pressure of 1000pounds which amounted to l0 pounds per square inch on the square inchsheet was applied. The platens were heated with l0 pounds per squareinch steam (approximately 240 F.) to cure the composition of Example 2.A molding time of 5 minutes was used at about 212 F. to cure thecomposition of Example 1. The sheets so formed and cured had somesparkle but were relatively clear and transparent.

Subsequent heat treatments are used to make the above sheets polarizing.The subsequent heat treatments are carried out at about 275 F. in thecase of the styrene monomer and at about 250 F. in the case of theacrylic composition. Much higher temperatures can be used, if desired.Temperatures up to 340 F. or more may be used. The time at which thecured sheets are exposed to further heat varies inversely with thetemperature used. Polarizing panels have been formed using heating timesof up to about 5 minutes or more.

The polarizing effect may be produced due to the vaporization of thenon-copolymerized or residual styrene or acrylic monomer which formstransition layers or pockets between some of the flakes and the resin.These transition layers have a low refractive index which provides thenecessary difference in index of refraction as compared to the glass sothat polarization results. In addition to the formation of transitionlayers, it is believed that the differential expansion between the resinand the flake which takes place in the subsequent heating steps maycause physical separation of some of the flakes from the resin duringheating. The resin expansion is about 50 times as great as the glassexpansion. During cooling the resin tends to contract much faster thanthe glass with which it is associated and, therefore, separation againwill be achieved between the resin and some of the flakes.

It has been observed that a sheet which is heated to higher and highertemperatures will have greater and greater changes in appearance. Anadded sparkle is achieved in the panel which might be described as asilvery appearance. The panel no longer remains clear and transparentbut becomes more milky or pearlescent. If desirable, rapid cooling canbe used to increase the pearlescent effect. The above phenomena may beprovided by a combination of the vaporization of the residual monomerand the differential expansivities of the materials, cured resin andglass.

It has been found that sheets having the silvery appearance arepolarizing. Postheating can be used to make panels polarizing or thepolarizing effect can be increased by postheating. Multiple postheatingsteps may be used, if it is desired.

The postheating temperature depends upon the particular resin or resinsbeing used and upon the type of monomer and its percentage included inthe previous cure cycle. Generally, shorter postheating periods may beused if the original cure temperatures are high. If low original curetemperatures are used, the postheating temperatures should be increasedto achieve the desired effect. If low cure temperatures are used, thenthe postheating must be carried out for long periods in order to get thepolarizing effect. Long postheating is sometimes undesirable since theresin may tend to discolor from the application of the heat. When usingpolyester resins, the desired curing conditions generally vary from l0to 50 pounds of steam (line gauge pressure) which provides temperaturesof from about 240 F. to 298 F. With such cure temperatures thepostheating temperatures will vary from about 250 F. to 400 F. in an airoven. If desirable, the flake and resin panels may be postheated betweenmetal sheets or the like in order to conduct and transmit the heat ofthe oven to the panel more effectively and to provide the desiredsurface configuration. After postheating, the panels are advantageouslycooled between Hat plates of wood, metal, or the like, to compress anysurface bubbles and to atten the panels or post form them to desiredshapes. A camber can be given a sheet in this manner to counteract theeffect of sag when the nished panels are installed.

Example 3 125 parts of polyester resin 70 parts of double thicknessflake 14 parts of styrene monomer 1.4 parts of benzoyl peroxide as acatalyst This mix was rolled out into a sheet between two sheets ofcellophane and then molded using pound steam for about 15 minutes. A atpanel was molded with the distribution of the flake within the resinbeing good. The panel was then afterheated at about 390 F. for 5 minutesto cause the sheet to be polarizing. As the panel was removed from thepostheating oven, it was quenched with water and then cooled betweensheet metal plates to maintain the panel in a perfectly flat condition.

Polyester resin with Hakes or platelets of glass may be cured attemperatures of about 235 F. and then postheated at from 300 F. to about400 F. and cooled in air or water to enhance the polarizingy propertiesof the panels. If desirable, one or both surfaces of the panels may beembossed to provide a random appearance. Some of the flakes will betilted to provide further polarization of light hitting at an anglenormal to the surface of the panel. In addition, the textured surfaceimproves the appearance and acts as a diffusing surface for lightpassing through the panel. Various surfaces including pebblegrained anddeep, random embossing may be used. It is generally desirable to havethe top surface of the panel smooth for ease of cleaning. A pebble-grainsurface upsets the linear or planar relationship of flakes and alsoprovides some polarization at 90 where normally no polarization would bepresent. The pebble-grain surface provides a uniform appearance andminimizes non-uniformity of internal flake distribution.

A specific example of a binder composition which includes a gassingagent is produced by mixing 100 parts of the polyester resin, 35 partsof monomeric styrene, and 3 parts of benzoyl peroxide. A polarizingelement according to the invention can be produced directly from thiscomposition merely by mixing glass flakes therewith, subjecting theresulting mixture to vacuum to remove entrapped air, and then curing anelement in the desired shape. The excess monomeric styrene acts as agassing agent during cure, and causes the desired interfacialdisturbance. The presence of an excess of catalyst enhances thedisturbance at the interfaces. The gassing effect can also be increasedby curing at a relatively high temperature, e.g., about 250 F. and thenpostheating to enhance the polarization still further.

It has been found with the work using polyester resins that at least aminimum cure must be given to provide suli'lcient hardness in the -resinbefore the postheating theatment so that an interfacial disturbance canbe caused by the heating and cooling. The postheating step makes thepolyester resin soft again and, if desired, the molding or the embossingmay take place during this postheating step, or a polarizing element ofany desired shape can be formed. Preferred polarizing effects areachieved when there are from 8 to 16, and most desirably about 12,disturbed interfaces, or gas pockets, on the average, in the thicknessof a polarizing sheet according to the invention. This means that from 4to 8, and most desirably, about 6, flakes, on the average, should havegas pockets disposed between their surfaces and the resinous binder inany Igiven thickness. In a specific instance, a panel produced from 40percent of glass flakes and 60 percent of a suitable binder resin, andhaving a thickness of 0.035", included, on the average, 100 flakes inthe thickness. In this case, from 4 to 8 percent of the flakes, and mostdesirably about `6 percent, should have gas pockets between both oftheir surfaces and the binder. If a thicker Sheet is produced, a smallerpercentage of interfacial disturbance is desired, while, if a thinnersheet is produced, a larger percentage of interfacial disturbance shouldbe produced. Similarly, if thinner or thicker glass flakes are used, alesser, or greater, respectively, percentage of interfacial disturbancewould be required for optimum results. A greater percentage ofinterfacial disturbance can be achieved by increasing the percentage ofstyrene or other monomer in the resinous binder, or by lpostcuring to ahigher temperature, for example. It will be appreciated that the glassakes which are adhered directly to the binder, or where there is nointerfacial disturbance, serve to reinforce the binder, while theremaining akes, which have interfacial disturbances, give the polarizingeffect. The same considerations are controlling when a mechanism otherthan popping is used to achive polarizing, e.g., a highor low-refractiveindex coating on glass flakes. The coated flakes or other mechanismprovide about the indicated number of interfacial changes, on theaverage, through any thickness.

It will be appreciated that the techniques of using a binder compositioncontaining a 4gassin-g agent, or a solvent, can be employed in theproduction of polarizing elements in the form of glass tlake papers.

Referring now to FIG. 4, the best presently known mode for practicingthe invention comprises the steps of mixing binder ingredients,deaerating the mixture, mixing glass ake-s with the deaerated bindermixture, sheeting the binder-flake mix, hot pressing the sheets toeffect cure of the binder, and heating the cured sheets to disturb thebinder-glass interfaces. These several steps are described in moredetail in the following example.

Example 4 A binder of the polymerizable, unsaturated polyester type isprepared by heating a charge consisting of 1.05 mols of ethylene glycol,0.8 mol of maleic anhydride, 0.2 mol of phthalic anhydride and an amountof hydroquinone equal to 0.04 percent of the charge to a termperature of230 C. in two hours, and holding the charge Iat a temperature between230 C. and 235 for 'lve additional hours. An part portion of theresulting reaction product, which is an unsaturated, polymerizablepolyester, is mixed with 20 parts of methyl methacrylate monomer, and a75 part portion of the resulting mixture is blended with 50 parts ofstyrene monomer, 0.3 part of vinyldirnethoxyethoxysilane, and 1.8 partsof benzoyl peroxide, using a suitably driven propeller for agitationuntil a uniform composition is achieved. The resulting composition isthen placed in a vacuum vessel 17 (FIG. 5), and a gasketed cover 1S stightly clamped onto the vessel 17. Vacuum i-s then applied to theclosed Vessel by connecting a line 19 to a vacuum pump, andprogressively reducing the pressure inside the vessel 17 to 27" ofmercury Vacuum. A vacuum of 5 of `mercury is maintained for two minutes,followed by a vacuum of 10 of mercury for two minutes, 15 of mercury foran additional two minutes, 20" of mercury for a further two minutes, andfinally a vacuum of 27" of mercury for fifteen tminutes.

The resulting deaerated binder composition is maintained at roomtemperature for two hours, and a 50 part portion thereof is then chargedto a container 20 of a mixer indicated generally at 21 (FIG. 6). Themixer 21 is provided with a paddle-type stirrer 22 which is mounted on ashaft 23 of a motor 24. A charge of 50 parts of glass flakes having athickness ranging from about 0.00010 to 0.00014 and an average majorsurface area of about 0.06 square inch is then prepared and added slowlyand stepwise to the binder charge in the container 20. After all of theglass akes have been added, the walls of the container 20 are scraped,and the mixer is run for about an additional half minute. Theconsistency of the resulting mixture is such that the mixture can 'bedescribed as spreadable, it being one which deforms readily underpressure.

Referring now to FIG. 7, the mixture is then charged to a hopper 25 inwhich it rests on sheets 26 of cellophane passed from supply rolls 28and over suitably driven cooperating calender rolls 29. The nip betweenthe calender rolls 29 is set from 0.032 to 0.035 to produce an ultimatecured sheet having a thickness of about 0.030 to about 0.035". The4binder-glass ake mixture is -formed into a sheet 30 between thecalender rolls 29, with cellophane cover sheets 26 above and below thesheet 30. Glass Hakes within the sheet 30 are oriented with their majorsurfaces generally parallel to the major surfaces of the sheet. Thesheet 30 and the cellophane sheets 26 are drawn onto a skid 31, and cutto a desired size. Various sheet materials other than cellophane canalso be used as cover sheets. For example, polyester films and parchmenthave -been used satisfactorily.

A suitably trimmed portion of the sheet 30 (designated 32 in FIG. 8) isthen placed on a lower steam heated platen 33 of a molding pressdesignated generally at 34. A resilient rubber gasket 35 is embedded inthe platen 33, and extends entirely around the periphery of a panelproduced in the press 34 from the sheet 32. As will be seen from FIG. 8,however, the sheet 32 extends beyond the gasket 35. A steam orelectrically heated platen 36 is suitably mounted for limited verticalmovement into and out of cooperating relationship with the platen 33.The platen 36 can be actuated by a suitable hydraulic cylinder (notillustrated). The sheet 32 is cured in the press 34, with the platens 33and 36 maintained at 210 F., and using a pressure yof 200 pounds persquare inch on the sheet, and a curing cycle of 3 to 5 minutes. Therubber gasket 35 is compressed when the platen 36 is lowered intocooperative relationship with the platen 33, and encloses the edges ofthe sheet 32 during cure thereof, so that the entire sheet is confined,and subjected to pressure. After curing is complete, the cellophane orother covers 26 are stripped from the sheet 32.

A cured sheet produced as described in the preceding paragraph, which isnearly transparent, is subjected to a further heat treatment in poppingapparatus indicated generally at 40 in FIG. 9. The apparatus 40comprises a table portion 41 over which a conveyor belt 42 is suitablydriven at variable speeds. Cured sheets, designated 43, are carried bythe conveyor belt 42 under resistance heating elements 44 which aresupported by a reflector 45, and suitably connected to any source for avariable voltage, for example the output side of a variable rheostat.Each of the heaters 44 in the structure shown in FIG. 9 comprises anouter tube 46 with a resistance coil 47 embedded therein, and packed inpowdered magnesium oxide. The sheets 43 are heated as they are movedunder the heating elements 44 by the conveyor 42, and to a temperaturewhich depends upon the power input to the resistance coils 47 and therate at which the sheets 43 are advanced. Excellent results areachieved, when the distance between the first and the last of theheating elements 44 is about four feet and when the elements 44 arepositioned about three inches above the sheets 43 by supplying power tothe coils 44 to maintain the temperature just above the sheets at about325 F., and by advancing the sheets 43 at a rate of about three feet perminute. The heating portion of the apparatus 40 is substantiallyenclosed by the reflector 45 and side plates 48, one of which is shownin FIG. 9. The etect of incidental air currents on heating of the sheets43 is minimized in this way. An oven at a temperature from about 250 F.to about 300 F. can also be used satisfactorily in place of theapparatus of FIG. 9.

The consequence of heat treating sheets 43 in the apparatus of FIG. 9can be observed visually. As is indicated above, the sheets 43 arenearly transparent. The

heat treatment, after curing, converts the sheets 43 to translucentpolarizing sheets 49.

If desired, parts of the sheets 43 can be protected against the heatingaction in the apparatus 40, so that such parts remain transparent andnon-polarizing, while the parts which are not so protected are madetranslucent and polarizing. For example, an insulating shield in adesired shape can be placed in a desired location on each of the sheets43 for this purpose. Where infra-red or the like heating is employed,instead of resistance heating as in the apparatus of FIG. 9, aphotographic negative having portions which are opaque or reflective toinfra-red or the like rays can be used for this purpose.

The inal sheet 49 is both a polarizing panel and a diffusing panel. Thediffusing characteristics of the panel 49 can be enhanced by using asone of the cover sheets 26, a parchment which has a patterned surface,or by mixing a small amount :of an opaque pigment with the binder.Mixing from about 0.1 percent to about 1.0 percent, preferably about 0.3percent of TiOz with the binder has been found to give good results.

A polarizing panel according to rthe invention, for example the panel 49shown in FIG. 9, is highly translucent to Iincident light wavestraveling normal to the major surfaces thereof, or at an angle up toabout 40 -to the normal, and substantially opaque to light wavesltraveling at angles of from 75 to 90 to the majo-r surfaces. `Opacityincreases as a direct function of angle of incidence for angles from 40to 75 to the major surlfaces. This feature of panels according to theinvention is important in t'he ceiling lighting eld Where a light remotefrom an observer, and being transmitted through `a panel according tothe invention, is not distracting, but a light immediately above theobserver lights his surroundings. This feature could also be used toadvantage in the installation of solar heating panels. Panels accordingto the invention could be used for this purpose, and positioned at suchan angle that they are highly transparent to heat and light energytransmitted by the sun during the winter season, when the position ofthe sun in the sky is relatively low, but are opaque to such energy whenthe sun is in a relatively high position in the sky during the summerseason. A panel produced according to the invention, and used 4in thismanner, enables `derivation of the benet of solar heating in the winter,while minimizing solar heating in the summer.

Although a description of thermosetting resins has been used in theexamples, thermoplastic resins may be used also in producing polarizingmembers. Resins such as the lacrylic resins, styrene, methylstyrene,imethylmethacrylate, methylethac-rylate, ethylmethacrylate,ethylethacrylate, and others may be used in a manner similar to thatdescribed for the thermosetting resins in the examples, but makingsuitable changes in processing techniques to compensate for thethermoplastic characteristics thereof.

Polarizing ele-ments can advantageously lbe used as window panes,headlight lenses, light bulbs, Venetian blind slats, light fixtures, 'inindustrial glazing, as street lighting -diffusers, as lamp shades, Iasinserts for glass block, or as coatings on the inside or outside oflight bulbs. -Particularly in the case .of ash bulbs which are to beused in colored photography, polarizing coatings are advantageousbecause color definitions in lm eicieny are improved `by polarizinglight. A particularly advantageous way for producing a light bulb,either a flash bulb or one used for ordinary lighting purposes, is byforming a slurry of a binder yand flake glass, and including a gassingagent, such as a solvent, an excess of styrene, an acrylic resin, or thelike, and applying this slurry to at least one surface of the bulb,either interior or exterior. The slurry can be applied by brushing,flowing, spraying, dipping or the like, and then can lbe hardened byheating. During hardening of the coating, the interfaces between binderand -glass are disturbed by the presence of the gassing agent, with Iheresult that the bulb is polarizing. Iff desired, suitable `dyes orpigments can be incorporated in a binder mix for use in producingpolarizing elements according to the invention, e.g., as described inExample 4, above. The dyes or pigments can be used to produce a panel ofa desied color, or to complement the characteristic yellow color of -theresin and the characteristic light yellow-green color of the glass toproduce a water clear panel.

Polarizing elements according to the invention can also be assembled toproduce articles which are useful both as polarizers and as diffusers,for various types of camera work. For" example, a structure indicatedgenerally at 50 in FIGS. 10 and ll comprises a reflector 51, a lightsource 52, and a combined polarizer and dif`t`user indicated generallyat 53. The combined polarizer and diffuser 53 is composed of a pluralityof relatively narrow slats 54, which are polarizing panels producedaccording to the invention. The slats 54 are fastened together, forexample by means -of a polymerizable polyester composition of the samegeneral type as the l.bindercontained in the slats, and so disposed thateach slat forms what may be described as a three-dimensional V with atleast one other slat. The angle between adjacent slats should be from 50to 60. As a consequence of the opacity of the slats 54 to light wavesincident at yan angle greater than about 40 to the normal, and of theangles between `adjacent slats 54, light waves from the source arerefluxed to a substantial extent before they ultimately strike one ofthe sla-ts 54 at an incident angle appropriate for transmission, so thatboth polarizing and diffusing are accomplished. It has been demonstratedthat interposinlg the structure 53 between a light source and a subject-to be photographed eliminates a substantial amount of glare andproduces sharper photographs.

A combined polarizing and ydiffusing device incorporating a panelproduced according to the invention, and somewhat similar to that shownin FIGS. and 1l, but significantly modified, is shown in FIG. 12. Thisstructure markedly changes the characteristics of light, so that a humaneye has been able to read much finer print illuminated with the lightthan the eye could read under -unpolarized light of the same intensity.The co-mbined polarizer and diffuser of FIG. 12 comprises a suitablereflectorized light source indicated generally at 55, and constructed inany suitable manner to throw a beam of generally parallel light waves inthe direction indicated in dotted lines onto an upper surface of apolarizing panel 56 produced according to the invention. The panel 56 isdisposed relative to the light source 55 so that the angle between theincident light waves and surface of the panel -is from about 55 to about60. Incident light on the panel 56 at such an angle is partiallytransmitted and partially reflected. One light wave is indicated by adotted line as being partly reflected from the -panel S6 against areflector 57, and 'back onto the panel 56. A part of this reflected waveis transmitted through the panel 56, while a further part thereof isagain reflected. It will be appreciate-d that other waves are similarlypartly reflected and partly transmitted, so that a refluxing action isachieved by the apparatus of FIG. 12, which action avoids excessivelosses of light which are characteristic of many polarizing assemblies,but also provides polarized light which, in a sense, has been diffused.

The paper-making procedures described above have also been employed toproduce papers 0.012" thick and 0.006" thick from a mixture of 50 partsof glass flakelets and 50 parts of bleached kraft, and to produce papers0.015 thick and 0.005I thick from a mixture of 75 parts of flake glassand parts of bleached kraft. The papers have increased dielectricstrengths and reduced moisture vapor transmissions by virtue of theinclusion therein of glass flakelets. Thin laminates produced therefrom,using polyester, epoxy, or low-loss phenolic binders have good tensileimpact and flexural strengths, low moisture absorption and transmission,and good surface hardness and flame and abrasion resistance. The papersor laminates produced therefrom are promising as surfacing sheets, e.g.,exterior, electrical laminates, road sign laminates, pipe materials, andin various decorative applications.

A fragment of a polarizing sheet 14 is shown in FIG. 3. The sheet 14 issimilar to the sheet 10 of FIGS. 1 and 2, except that the gas pockets 13and omitted. Instead, the sheet 14 is composed of glass flakelets 15embedded in a binder resin 16. Either the flakes or a coating applied totheir surfaces has a refractive index differing from that of the binderresin 16 by at least 0.2.

The relative proportions between a binder and glass flakes in apolarizing panel according to the invention can be varied within broadlimits. For example, such a panel has been produced using as little as10 percent of flakes with percent of the binder, and as much as 75percent of flakes with 25 percent of the binder, and as much as 90percent of flakes could be used with l0 percent of the binder, ifdesired. An essen-tial feature of such a panel is the provision of asubstance having `a refractive index differing from that of the glass byat least 0.2, as discussed above.

The terms percent and parts as hereinbefore used, and as used in theappended claims, refer to percent and parts by weight, unless otherwiseindicated.

It will be apparent that various changes and modifications can be madefrom the specific details discussed above without departing from thespirit of the attached claims.

What I claim is:

1. A method for producing a polarizing element comprising a bindercomposition and -a plurality of glass flakelets dispersed therein, thepolarizing element having randomly disposed interfacial disturbanceregions uniformly distributed throughout the element in which the bindercomposition is separated from the surfaces of the flakelets, theinterfacial disturbance regions dening pockets having a first majorsurface bounded by at least one flakelet and a second opposed majorsurface bounded by the binder composition, said method consistingessentially of the steps of mixing glass flakelets with a dispersion ofa resinous material hardenable at elevated temperatures to a transparentmass and an` excess of a volatile substance to produce a mixture whichconsists essentially of the glass flakelets, the resinous material andthe volatile substance, three dimensionally confining the mixture undera pressure sufficient to prevent excessive volatilization and to preventthe formation of massive gas bubbles, and heating the mixture whileunder the confining pressure to harden the resinous material and tovaporize the volatile substance to a degree necessary to cause theinterfacial disturbance regions between glass flakelets and the resinousmaterial and to render the mixture polarizing.

2. A method as claimed in claim 1 wherein the volatile substance is apolymerizable monomeric material.

3. A method as claimed in claim 1 wherein the volatile substance is asolvent for the resinous material.

4. A method as claimed in claim 1 wherein the volatile substance iswater.

5. A method for producing a polarizing element comprising a bindercomposition and a plurality of glass flakelets dispersed therein, thepolarizing element having randomly disposed interfacial disturbanceregions uniformly distributed throughout the element in which bindercomposition is separated from the surfaces of the flakelets, theinterfacial disturbance regions defining pockets having a first majorsurface bounded by at least one flakelet and a second opposed majorsurface bounded by the binder composition, said method consistingessentially of the steps of mixing about 50 parts of glass flakeletswith a binder composition which consists essentially of 100 parts of apolyester resin curable by addition polymerization to a hard,light-transmitting mass, 50 parts of styrene monomer and `about 1 partof benzoyl peroxide, forming the resulting mixture into a desired shape,three dimensionally confining the mixture under a pressure sufficient toprevent excessive volatilization and to prevent the formation of massivegas bubbles, and applying heat to the mixture While under the continingpressure to convert the polyester to a hardened condition `and tovaporize a portion of the styrene monomer to cause the interfacialdisturbance regions between glass akelets and the binder composition andto render the mixture polarizing.

References Cited by the Examiner UNITED STATES PATENTS Gates et a1.264-55 Peumer 264-55 Brown et al. 264-2 Marks et a1. 264-2

1. AMETHOD FOR PRODUCING A POLARIZING ELEMENT COMPRISING A BINDERCOMPOSITION AND A PLURALITY OF GLASS FLAKELETS DISPERSE THEREIN, THEPOLARIZING ELEMENT HAVING RANDOMLY DISPOSED INTERFACIAL DISTRUBANCEREGIONS UNIFORMLY DISTRIBUTED THROUGOUT THE ELEMENT IN WHICH THE BINDERCOMPOSITION IS SEPARATED FROM THE SURFACES OF THE FLAKELETS, THEINTERFACIAL DISTURBANCE REGIONS DEFINING POCKETS HAVING A FIRST MAJORSURFACE BOUNDED BY AT LEAST ONE FLAKELET AND A SECOND OPPOSED MAJORSURFACE BOUNDED BY THE BINDER COMPOSITION, SAID METHOD CONSISTINGESSENTIALLY OF THE STEPS OF MIXING GLASS FLAKELETS WITH A DISPERSION OFA RESIONOUS MATERIAL HARDENABLE AT ELEVATED TEMPERATURES TO ATRANSPARENT MASS AND AN EXCESS OF A VOLATILE SUBSTANCE TO PRODUCE AMIXTURE WHICH CONSISTS ESSENTIALLY OF THE GLASS FLAKELETS, THE RESINOUSMATERIAL AND THE VOLATILE SUBSTANCE, THREE DIMENSIONALLY CONFINING THEMIXTURE UNDER A PRESSURE SUFFICIENT TO PREVENT EXCESSIVE VOLATILIZATIONAND TO PREVENT THE FORMATION OF MASSVE GASBUBBLES, AND HEATING THEMIXTURE WHILE UNDER THE CONFINING PRESSURE TO HARDEN THE RESIONOUSMATERIAL AND TO VAPORIZE THE VOLATILE SUBSTANCE TO A DEGREE NECESSARY TOCAUSE THE INTERFACIAL DISTURBANCE REGIONS BETWEEN GLASS FLAKELETS ANDTHE RESIONOUS MATERIAL AND TO RENDER THE MIXTURE POLARIZING.